Stack design and dispersion: A large thermal power plant typically discharges flue gas through its chimney at about what velocity (metres per second)?

Introduction:
Flue-gas exit velocity from stacks influences plume rise, dispersion, and the potential for stack-tip downwash. Power plant stacks are designed to balance acceptable pressure drop with sufficient momentum for effective dispersion.

Given Data / Assumptions:

• Utility-scale thermal power plant with tall chimney.
• Normal operating draft and induced-draft fans.
• Velocity values need only be order-of-magnitude correct.

Concept / Approach:
Typical stack exit velocities range around 10–20 m/s. Much lower values cause poor dispersion and downwash; extremely high velocities would demand excessive fan power and are rarely practical. Thus, a representative benchmark value is approximately 10 m/s for big plants.

Step-by-Step Solution:
Consider energy cost and dispersion needs → moderate velocities preferred.Industry practice → design around ~10–20 m/s.Choose 10 m/s as the nearest standard value among the options.

Verification / Alternative check:
Environmental permitting guidelines often cite stack exit velocity targets near this range to limit downwash and meet dispersion modeling assumptions.

Why Other Options Are Wrong:

• 0.5 or 2 m/s: Too low; risk of downwash and re-entrainment.
• 50 m/s: Unusually high and inefficient for large stacks.
• 500 m/s: Physically unrealistic for stack flows.

Common Pitfalls:
Confusing flue-gas duct velocities (which can be higher) with final stack exit velocities constrained by environmental and energy considerations.

Final Answer:
10